Image forming apparatus

ABSTRACT

An image forming apparatus includes a plurality of light emitting elements, a detector, a controller, and an image forming unit. The light emitting elements are arranged in at least one row. The detector is configured to detect a number of light emitting elements that emit light according to image data. The controller is configured to control a driving voltage for driving the light emitting elements based on the number of light emitting elements detected. The image forming unit is configured to form an image based on light emission of the light emitting elements according to the image data.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.17/016,213, filed Sep. 9, 2020, the entire contents of which areincorporated herein by reference.

FIELD

Exemplary embodiments described herein relate to an image formingapparatus.

BACKGROUND

Electrophotographic printers (hereinafter, printers) including a printhead are widespread. The print head includes a plurality of lightemitting elements such as light emitting diode (LED) or organic lightemitting diode (OLED). For example, the print head is provided withlight emitting elements corresponding to 15400 pixels, a direction inwhich light emitting elements are arranged corresponds to a mainscanning direction, and a direction orthogonal to the main scanningdirection corresponds to a sub-scanning direction. The printer exposes aphotoreceptor drum with light emitted from the plurality of lightemitting elements, and prints an image corresponding to a latent imageformed in the photoreceptor drum on a sheet which is a recording papersheet.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating an example of a positional relationshipbetween a photoreceptor drum and a print head in an image formingapparatus according to an embodiment;

FIG. 2 is a view illustrating an example of a transparent board includedin the print head according to the embodiment;

FIG. 3 is a view illustrating an example of a layout of light emittingelements and driving circuits of the print head according to theembodiment;

FIG. 4 is a view illustrating an example of a section of the transparentboard of the print head according to the embodiment;

FIG. 5 is a view illustrating a connection example between a controlboard and the print head according to the embodiment;

FIG. 6 is a view describing an example of a structure of the lightemitting element of the print head according to the embodiment;

FIG. 7 is a schematic diagram illustrating an example of a circuitconfiguration including a DRV circuit for driving the light emittingelement according to the embodiment, the light emitting element thatemits light by the DRV circuit, and a switch that switches currentsupply to the light emitting element;

FIG. 8 is a diagram illustrating an example of a head circuit block ofthe print head according to the embodiment;

FIG. 9 is a view illustrating an example of an image forming apparatusto which a print head according to a first embodiment is applied;

FIG. 10 is a block diagram illustrating an example of a control systemof the image forming apparatus according to the embodiment;

FIG. 11 is a flowchart illustrating an example of driving voltagecontrol by the image forming apparatus according to the embodiment;

FIG. 12 is a diagram illustrating a relationship between the number oflight emitting elements that emit light and a light quantity decreaserate in an optical head of the image forming apparatus according to theembodiment;

FIG. 13 is a diagram illustrating an example of a relationship between ahalftone image (when the number of light emitting elements that emitlight is constant) formed by the image forming apparatus according tothe embodiment, and a voltage and a driving current supplied to theprint head in response to the formation of the halftone image;

FIG. 14 is a diagram illustrating a non-application example of thedriving voltage control according to the embodiment;

FIG. 15 is a diagram illustrating an application example of the drivingvoltage control according to the embodiment;

FIG. 16 is a diagram illustrating an example of an effect of suppressinga light quantity decrease due to an increase in voltage applied to theoptical head of the image forming apparatus according to the embodiment;and

FIG. 17 is a block diagram illustrating a modification example of thecontrol system of the image forming apparatus according to theembodiment.

DETAILED DESCRIPTION

An image forming apparatus according to an embodiment includes a printhead, a detection section, a controller, and a power source section. Theprint head includes one or a plurality of light emitting element rowscomposed of a plurality of light emitting elements. The detectionsection detects the number of light emitting elements that emit lightaccording to image data. The controller controls a driving voltage fordriving the light emitting elements based on a detection result. Thepower source section supplies the driving voltage to the print head.

Hereinafter, an example of the image forming apparatus according to theembodiment will be described with reference to the drawings. In eachdrawing, the same reference numerals will be given to the sameconfigurations. The image forming apparatus is a printer, a copyingmachine, or a multifunctional peripheral (MFP). In the embodiment, animage forming apparatus corresponding to the MFP will be described.

Configuration of Print Head

An example of the configuration of the print head applied to the imageforming apparatus according to the embodiment will be described withreference to FIGS. 1 through 8.

FIG. 1 is a view illustrating an example of a positional relationshipbetween a photoreceptor drum and a print head applied to an imageforming apparatus according to an embodiment.

An image forming apparatus 100 includes a photoreceptor drum 17 and aprint head 1 illustrated in FIG. 1. The print head 1 is disposed so asto face the photoreceptor drum 17. The photoreceptor drum 17 rotates ina direction of an arrow illustrated in FIG. 1. A rotational direction ofthe photoreceptor drum 17 is called a sub-scanning direction, and adirection orthogonal to the sub-scanning direction is called a mainscanning direction. The photoreceptor drum 17 is uniformly charged by acharging device (e.g., charger) and exposed with the light from theprint head 1, and a potential (e.g., voltage potential) of the exposedpart decreases. In other words, the image forming apparatus 100 controlsthe light emission of the print head 1 and forms an electrostatic latentimage on the photoreceptor drum 17. Controlling the light emission ofthe print head 1 is controlling the timing of light emission andlight-off (non-light emission) of the print head 1.

The print head 1 includes a light emitting section 10 and a rod lensarray 12. The light emitting section 10 includes a transparent board 11.For example, the transparent board 11 is a glass board that transmitslight. A plurality of light emitting element rows 13 composed of aplurality of light emitting elements 131 such as LEDs or OLEDs areformed on the transparent board 11.

As illustrated in FIG. 1, two rows of a first light emitting element row1301 and a second light emitting element row 1302, are arranged inparallel. The rod lens array 12 collects light from each light emittingelement 131 of the two rows of the first light emitting element row 1301and the second light emitting element row 1302 on the photoreceptor drum17. Accordingly, an image line corresponding to the light emission ofthe light emitting elements 131 is formed on the photoreceptor drum 17.In the embodiment, a case where the print head 1 includes a plurality oflight emitting element rows 13 will be described, but it is alsopossible for the print head 1 to include only a single light emittingelement row 13. Further, the print head 1 may also include a gap spacer121. The gap spacer 121 maintains a predetermined distance between thetransparent board 11 and the photoreceptor drum 17. FIG. 2 is a viewillustrating an example of the transparent board included in the printhead according to the embodiment.

As illustrated in FIG. 2, at a center portion of the transparent board11, two light emitting element rows 13 (e.g., the first light emittingelement row 1301 and the second light emitting element row 1302) areformed along a longitudinal direction of the transparent board 11. Inthe vicinity of the light emitting element row 13, driving circuit rows14 (e.g., a first driving circuit row 1401 and a second driving circuitrow 1402) for driving (e.g., causing light emission of) each lightemitting element are formed. Hereinafter, “driving” is referred to as“DRV”. In FIG. 2, the DRV circuit rows 14 for driving (e.g., causinglight emission of) the light emitting elements 131 are disposed on bothsides of the two light emitting element rows 13, but the DRV circuitrows 14 may be arranged on one side.

An integrated circuit (IC) 15 is disposed at an end portion of thetransparent board 11. In addition, the transparent board 11 includes aconnector 16. The connector 16 electrically connects the print head 1 toa control system of a printer, a copying machine, or a multifunctionalperipheral. This connection enables electric power supply, head control,image data transfer, and the like. A board for sealing the lightemitting element rows 13, the DRV circuit rows 14, and the like so asnot to come into contact with outside air is attached to the transparentboard 11. Furthermore, when it is difficult to mount the connector onthe transparent board, a flexible printed circuit (FPC) may be connectedto the transparent board and electrically connected to the controlsystem.

FIG. 3 is a view illustrating an example of a layout of the lightemitting elements and the driving circuits of the print head accordingto the embodiment.

As illustrated in FIG. 3, the light emitting section 10 of the printhead 1 includes both the light emitting element rows 13 in which theplurality of light emitting elements 131 are arranged and the drivingcircuit rows 14 in which the plurality of driving circuits 140 arearranged. The driving circuits 140 cause the light emitting elements 131connected to the respective driving circuits 140 to emit light based onsignals (e.g., a sample and hold signal 21, a light emission levelsignal 22, a light emission ON signal 26, and a light emission OFFsignal 27, which will be described later) of a wiring 145.

FIG. 4 is a view illustrating an example of a section of the transparentboard of the print head according to the embodiment.

As illustrated in FIG. 4, the light emitting section 10 of the printhead 1 includes the plurality of light emitting elements 131, theplurality of driving circuits 140, and the wirings 145 which aredisposed to oppose a reference surface 1101 of the transparent board 11.In addition, the light emitting section 10 includes a sealing glass1102. The plurality of light emitting elements 131, the plurality ofdriving circuits 140, and the wirings 145 are disposed in the spacesurrounded by the transparent board 11 and the sealing glass 1102. Thelight from the light emitting element 131 passes through the transparentboard 11 and is emitted toward the photoreceptor drum 17.

FIG. 5 is a view illustrating a connection example between the controlboard and the print head according to the embodiment.

As illustrated in FIG. 5, the image forming apparatus 100 includes acontrol board 101, and the control board 101 includes a power sourcesection 102. The power source section 102 supplies a power sourcevoltage VDDa to both ends of the print head 1 via a harness 104. Therelationship between the number of light emitting elements 131 that emitlight and a light quantity decrease rate will be described later.

FIG. 6 is a diagram describing an example of a structure of the lightemitting element of the print head according to the embodiment. In FIG.6, the sealing glass 1102 is omitted.

For example, the light emitting element 131 is an organicelectroluminescence (organic EL). As illustrated in FIG. 6, the lightemitting element 131 includes a hole transport layer 1311, a lightemitting layer 1312, and an electron transport layer 1313, and is incontact with and sandwiched between an electrode (+) 1321 and anelectrode (−) 1323 insulated by an insulating layer 1322. In the firstembodiment, for example, the light emitting layer 1312 is an organic EL.The electrode (−) 1323 has a structure for reflecting light emitted fromthe light emitting layer 1312. With this structure, the light emittedfrom the light emitting layer 1312 is output to the transparent board 11side.

FIG. 7 is a schematic diagram illustrating an example of a circuitconfiguration including the DRV circuit for driving the light emittingelement according to the embodiment, the light emitting element thatemits light by the DRV circuit, and a switch that switches currentsupply to the light emitting element.

The DRV circuit is composed of a low-temperature polysilicon thin filmtransistor. The sample and hold signal 21 becomes an “L” level whenlight emission intensity of the light emitting element 131 connected tothe DRV circuit 140 is changed. When the sample and hold signal 21becomes an “L” level, a voltage of a capacitor 142 changes according tothe voltage of the light emission level signal 22. In other words, thecapacitor 142 holds a potential that changes according to correctiondata which will be described below.

When the sample and hold signal 21 becomes an “H” level, the voltage ofthe capacitor 142 is held. Even when the voltage of the light emissionlevel signal 22 changes, the voltage level of the capacitor 142 does notchange. A current according to the voltage held in the capacitor 142flows through the light emitting element 131 connected to a signal lineI of the DRV circuit 140. In other words, the light emitting element 131emits light according to the potential of the capacitor. A predeterminedlight emitting element 131 is selected from the plurality of lightemitting elements 131 included in the light emitting element row 13 bythe sample and hold signal 21, the light emission intensity isdetermined by the light emission level signal 22, and the light emissionintensity can be maintained.

Further, a switch 144 is connected to the DRV circuit 140. The switch144 switches between supply and non-supply (ON and OFF of currentsupply) of current supply to the light emitting element 131. When theswitch 144 is closed by the light emission ON signal 26, a current flowsthrough the light emitting element 131 and the light emitting element131 emits light. When the switch 144 is opened by the light emission OFFsignal 27, no current flows through the light emitting element 131 andthe light emitting element 131 is turned off. FIG. 8 is a diagramillustrating an example of a head circuit block of the print headaccording to the embodiment.

As illustrated in FIG. 8, the light emitting section 10 includes thehead circuit block including the IC 15. The IC 15 includes a lightemitting element address counter 151, a decoder 152, a digital to analog(D/A) conversion circuit 153, a light quantity correction memory 154, alight emission ON or OFF instruction circuit 155, and the like. Thelight emitting element address counter 151, the decoder 152, the D/Aconversion circuit (e.g., converter) 153, the light quantity correctionmemory 154, and the light emission ON or OFF instruction circuit 155supply the sample and hold signal 21, the light emission level signal22, the light emission ON signal 26, and the light emission OFF signal27 which are described in advance to the DRV circuit 140 and the like.

As illustrated in FIG. 8, the light emitting elements 131 arerespectively connected to the DRV circuits 140. Each of the individualDRV circuits 140 supplies individual currents to corresponding one ofthe individual light emitting elements 131. The D/A conversion circuit153 is connected to the first DRV circuit row 1401 connected to thefirst light emitting element row 1301. Similarly, the D/A conversioncircuit 153 is connected to the DRV circuit row 1402 connected to thesecond light emitting element row 1302.

The light quantity correction memory 154 stores the correction dataaccording to the current that flows through each light emitting element131. A horizontal synchronizing signal 24 and an image data writingclock C are input to the light emitting element address counter 151 viathe connector 16. The horizontal synchronizing signal 24 resets a countvalue of the light emitting element address counter 151. The lightemitting element address counter 151 outputs a light emitting elementaddress signal 25 synchronized with the image data writing clock C.

Image data 31 and the light emitting element address signal 25 outputfrom the light emitting element address counter 151 are input to thelight quantity correction memory 154. The light emitting element addresssignal 25 output from the light emitting element address counter 151 isinput to the decoder 152. The decoder 152 outputs the sample and holdsignal 21 corresponding to the light emitting element 131 designated bythe light emitting element address signal 25. The light quantitycorrection memory 154 outputs correction data 33 corresponding to thelight emitting element 131 designated by the light emitting elementaddress signal 25. The correction data 33 output from the light quantitycorrection memory 154 is input to the D/A conversion circuit 153. TheD/A conversion circuit 153 outputs the voltage of the light emissionlevel signal 22 based on the correction data 33. The voltage of thelight emission level signal 22 is sampled and held in the capacitor 142of the DRV circuit 140. The sampling and holding in the capacitor 142are periodically performed.

Configuration of Image Forming Apparatus

FIG. 9 is a diagram illustrating an example of an image formingapparatus to which the print head according to the first embodiment isapplied. FIG. 9 illustrates an example of a quadruple tandem-type colorimage forming apparatus, but the print head 1 of the embodiment can alsobe applied to a monochrome image forming apparatus.

As illustrated in FIG. 9, for example, an image forming apparatus 100includes an image forming unit 1021 that forms a yellow (Y) image, animage forming unit 1022 that forms a magenta (M) image, an image formingunit 1023 that forms a cyan (C) image, and an image forming unit 1024that forms a black (K) image. The image forming units 1021, 1022, 1023,and 1024 form yellow, cyan, magenta, and black images, respectively, andtransfer the images to a transfer belt 103. Accordingly, a full-colorimage is formed on the transfer belt 103.

The image forming unit 1021 that forms a yellow (Y) image includes aprint head 1001, and the print head 1001 includes a light emittingsection 1011 and a rod lens array 1201. Furthermore, the image formingunit 1021 includes an electrostatic charger 1121, the print head 1001, adeveloping device (e.g., a developer) 1131, a transfer roller 1141, anda cleaner 1161 around a photoreceptor drum 1701. The print head 1001corresponds to the print head 1, the light emitting section 1011corresponds to the light emitting section 10, the rod lens array 1201corresponds to the rod lens array 12, the photoreceptor drum 1701corresponds to the photoreceptor drum 17, and the description thereofwill be omitted.

The image forming unit 1022 that forms a magenta (M) image includes aprint head 1002, and the print head 1002 includes a light emittingsection 1012 and a rod lens array 1202. Furthermore, the image formingunit 1022 includes an electrostatic charger 1122, the print head 1002, adeveloping device (e.g., a developer) 1132, a transfer roller 1142, anda cleaner 1162 around a photoreceptor drum 1702. The print head 1002corresponds to the print head 1, the light emitting section 1012corresponds to the light emitting section 10, the rod lens array 1202corresponds to the rod lens array 12, the photoreceptor drum 1702corresponds to the photoreceptor drum 17, and the description thereofwill be omitted. The image forming unit 1023 that forms a cyan (C) imageincludes a print head 1003, and the print head 1003 includes a lightemitting section 1013 and a rod lens array 1203. Furthermore, the imageforming unit 1023 includes an electrostatic charger 1123, the print head1003, a developing device (e.g., a developer) 1133, a transfer roller1143, and a cleaner 1163 around a photoreceptor drum 1703. The printhead 1003 corresponds to the print head 1, the light emitting section1013 corresponds to the light emitting section 10, the rod lens array1203 corresponds to the rod lens array 12, the photoreceptor drum 1703corresponds to the photoreceptor drum 17, and the description thereofwill be omitted.

The image forming unit 1024 that forms a black (K) image includes aprint head 1004, and the print head 1004 includes a light emittingsection 1014 and a rod lens array 1204. Furthermore, the image formingunit 1024 includes an electrostatic charger 1124, the print head 1004, adeveloping device (e.g., a developer) 1134, a transfer roller 1144, anda cleaner 1164 around a photoreceptor drum 1704. The print head 1004corresponds to the print head 1, the light emitting section 1014corresponds to the light emitting section 10, the rod lens array 1204corresponds to the rod lens array 12, the photoreceptor drum 1704corresponds to the photoreceptor drum 17, and the description thereofwill be omitted.

The electrostatic chargers 1121, 1122, 1123, and 1124 uniformly chargethe photoreceptor drums 1701, 1702, 1703, and 1704, respectively. Theprint heads 1001, 1002, 1003, and 1004 expose the photoreceptor drums1701, 1702, 1703, and 1704, respectively, by the light emission of thelight emitting elements 131 of the first light emitting element row 1301and the second light emitting element row 1302, and form electrostaticlatent images on the photoreceptor drums 1701, 1702, 1703, and 1704. Thedeveloping device 1131, the developing device 1132, the developingdevice 1133, and the developing device 1134 respectively stick (develop)a yellow toner, a magenta toner, a cyan toner, and a black toner on theelectrostatic latent image parts of the respective photoreceptor drums1701, 1702, 1703, and 1704.

The transfer rollers 1141, 1142, 1143, and 1144 transfer the tonerimages developed on the photoreceptor drums 1701, 1702, 1703, and 1704to the transfer belt 103. The cleaners 1161, 1162, 1163, and 1164 cleantoners which are not transferred and left on the photoreceptor drums1701, 1702, 1703, and 1704, and are in a standby state for the nextimage formation.

A paper sheet (e.g., an image forming medium) 201 having a first size(e.g., a small size) is stored in a paper sheet cassette 1171 which is apaper sheet supply unit. A paper sheet (image forming medium) 202 havinga second size (e.g., a large size) is stored in a paper sheet cassette1172 which is a paper sheet supply unit.

A toner image is transferred to the paper sheet 201 or 202, which ispicked up from the paper sheet cassette 1171 or 1172, from the transferbelt 103 by a transfer roller pair 118 which is a transfer unit. Thepaper sheet 201 or 202 to which the toner image is transferred is heatedand pressurized by a fixing roller 120 of a fixing section 119. Thetoner image is firmly fixed on the paper sheet 201 or 202 due to heatingand pressurizing by the fixing roller 120. By repeating theabove-described process operation, the image forming operation iscontinuously performed.

FIG. 10 is a block diagram illustrating an example of a control systemof the image forming apparatus according to the embodiment.

As illustrated in FIG. 10, the image forming apparatus 100 includes thecontrol board 101. The control board 101 includes the power sourcesection 102, an image reading section 171, an image processing section172, an image forming section 173, a controller 174, a read only memory(ROM) 175, a random access memory (RAM) 176, a non-volatile memory 177,a communication I/F 178, a control panel 179, page memories 1801, 1802,1803, and 1804, a light emission controller 183, and an image data bus184. Furthermore, the image forming apparatus 100 includes a color shiftsensor 181, and a mechanical control driver 182. The image formingsection 173 includes image forming units 1021, 1022, 1023, and 1024. Thepower source section 102 supplies a driving voltage to both ends of theprint heads 1001, 1002, 1003, and 1004 of the image forming section 173via the harness 104.

The ROM 175, the RAM 176, the non-volatile memory 177, the communicationI/F 178, the control panel 179, the color shift sensor 181, themechanical control driver 182, and the light emission controller 183 areconnected to the controller 174. The image reading section 171, theimage processing section 172, the controller 174, the page memories1801, 1802, 1803, and 1804 are connected to the image data bus 184. Thepage memories 1801, 1802, 1803, and 1804 output Y, M, C, and K imagedata 31 respectively. The light emission controller 183 is connected tothe page memories 1801, 1802, 1803, and 1804, and the Y image data 31from the page memory 1801, the M image data 31 from the page memory1802, the C image data 31 from the page memory 1803, and the K imagedata 31 from the page memory 1804 are input. The print heads 1001, 1002,1003, and 1004 are connected to the light emission controller 183 so asto correspond to the respective pieces of the image data 31. The lightemission controller 183 inputs the respective pieces of the image data31 into the print heads 1001, 1002, 1003, and 1004 corresponding to therespective pieces of the image data 31.

The controller 174 includes one or more processors and controlsoperations such as image reading, image processing, and image formationaccording to various programs stored in at least one of the ROM 175 andthe non-volatile memory 177.

Further, the controller 174 inputs image data of a test pattern onto thepage memories 1801, 1802, 1803, and 1804 and forms the test pattern. Thecolor shift sensor 181 detects the test pattern formed on the transferbelt 103 and outputs a detection signal to the controller 174. Thecontroller 174 can recognize a positional relationship of test patternsof each color from the input of the color shift sensor 181. Furthermore,the controller 174 selects the paper sheet cassette 1171 or 1172 forfeeding paper sheets on which an image is to be formed through themechanical control driver 182.

The ROM 175 stores various programs or the like necessary for thecontrol of the controller 174. The various programs include a lightemission control program of the print head. The light emission controlprogram is a program for controlling the timing of light emission andlight-off (non-light emission) based on the image data.

The RAM 176 temporarily stores data necessary for the control of thecontroller 174. The non-volatile memory 177 stores a part or all ofvarious programs, various parameters, and the like.

The mechanical control driver 182 controls an operation of a motor orthe like necessary for printing according to the instruction of thecontroller 174. The communication I/F 178 outputs various pieces ofinformation to the outside and also inputs various pieces of informationfrom the outside. For example, the communication I/F 178 acquires imagedata including a plurality of image lines. The image forming apparatus100 prints the image data acquired via the communication I/F 178 by theprint function. The control panel 179 receives operation inputs from auser and a service personnel.

The image reading section 171 optically reads the image of a document,acquires the image data including the plurality of image lines, andoutputs the image data to the image processing section 172. The imageprocessing section 172 executes various types of image processing suchas correction with respect to the image data input via the communicationI/F 178 or the image data from the image reading section 171. The pagememories 1801, 1802, 1803, and 1804 store the image data processed bythe image processing section 172. The controller 174 edits the imagedata on the page memories 1801, 1802, 1803, and 1804 so as to match aprint position or the print head. The image forming section 173 forms animage based on the image data stored in the page memories 1801, 1802,1803, and 1804. In other words, the image forming section 173 forms animage based on the light emission (light emission and light-off state)of each light emitting element 131 according to the image data.

The light emission controller 183 includes one or more processors andcontrols the light emission of the light emitting element 131 based onthe image data according to various programs stored in at least one ofthe ROM 175 and the non-volatile memory 177. The light emissioncontroller 183 includes a light emitting element number detectionsection (e.g., a detector) 1831 and a driving voltage controller 1832.The light emitting element number detection section 1831 detects thenumber of light emitting elements 131 that emit light according to theimage data before the light emitting element 131 emits light accordingto the image data. For example, the light emitting element numberdetection section 1831 detects a proportion of the light emittingelements that emit light in units of one or a plurality of lightemitting element rows. While a case where all of the light emittingelements 131 of the light emitting element row 13 (the first lightemitting element row 1301 and the second light emitting element row1302) emit light is defined as 100%, the light emitting element numberdetection section 1831 detects whether or not the proportion of thelight emitting elements that emit light is equal to or less than 20%,whether or not the proportion is equal to or less than 40%, whether ornot the proportion is equal to or less than 60%, and whether or not theproportion is equal to or less than 80%. In addition, the detectionproportion is an example, and any proportion can be applied. Inaddition, when the light emission timing (e.g., phase) of the lightemitting element 131 differs depending on the disposition position inthe main scanning direction, the light emitting element number detectionsection 1831 may detect the number of light emitting elements 131 thatemit light simultaneously from the image data and the dispositionposition of the light emitting elements 131 that emit light.Furthermore, the proportion of the light emitting elements that emitlight may be detected from the number of light emitting elements 131that emit light at the same time.

The driving voltage controller 1832 controls a driving voltage fordriving the light emitting element 131 based on the detection result ofthe number of light emitting elements that emit light. In other words,the driving voltage controller 1832 controls the driving voltagesupplied to both ends of the print head 1 from the power source section102 based on the detection result of the number of light emittingelements that emit light. For example, the driving voltage controller1832 changes (increases or decreases) the driving voltage supplied tothe print head 1 based on the proportion of the light emitting elementsthat emit light.

Driving Voltage Control

FIG. 11 is a flowchart illustrating an example of driving voltagecontrol by the image forming apparatus according to the embodiment.

The communication interface 178 receives the image data and outputs thereceived image data. Otherwise, the image reading section 171 reads thedocument image and outputs the read image data. The controller 174executes printing based on the image data (ACT 101, YES).

For example, when the image data corresponding to each color is received(that is, in a case of color printing), the image processing section 172converts the image data corresponding to each color into raster data,and loads the converted raster data to the page memories 1801, 1802,1803, and 1804. The page memories 1801, 1802, 1803, and 1804 output theimage data corresponding to one line.

The light emitting element number detection section 1831 detects thenumber of light emitting elements 131 that emit light based on the imagedata corresponding to one line (ACT 102). In other words, the lightemitting element number detection section 1831 detects the number oflight emitting elements 131 that correspond to each color and emitlight. Further, the driving voltage controller 1832 controls the drivingvoltage for driving the light emitting element 131 based on thedetection result of the number of light emitting elements 131 that emitlight (ACT 103 to ACT 111).

For example, when the proportion of the light emitting elements 131 thatemit light is equal to or less than 20% (first proportion) (ACT 103,YES), the driving voltage controller 1832 controls the driving voltageto the reference voltage (VDD) and supplies the reference voltage. Whenthe driving voltage is already controlled to the reference voltage, thedriving voltage controller 1832 does not change the driving voltage, andwhen the driving voltage is controlled to a driving voltage higher thanthe reference voltage, the driving voltage controller 1832 decreases thedriving voltage to the reference voltage (ACT 104).

When the proportion of the light emitting elements 131 that emit lightexceeds 20% (ACT 103, NO) and is equal to or less than 40% (secondproportion) (ACT 105, YES), the driving voltage controller 1832 changesthe driving voltage to a first driving voltage which is higher than thereference voltage by 2%, and supplies the first driving voltage (ACT106). When the proportion of the light emitting elements 131 that emitlight exceeds 40% (ACT 105, NO) and is equal to or less than 60% (thirdproportion) (ACT 107, YES), the driving voltage controller 1832 changesthe driving voltage to a second driving voltage which is higher than thereference voltage by 4%, and supplies the second driving voltage (ACT108).

When the proportion of the light emitting elements 131 that emit lightexceeds 60% (ACT 107, NO) and is equal to or less than 80% (fourthproportion) (ACT 109, YES), the driving voltage controller 1832 changesthe driving voltage to a third driving voltage which is higher than thereference voltage by 6%, and supplies the third driving voltage (ACT110). When the proportion of the light emitting elements 131 that emitlight exceeds 80% (ACT 109, NO), the driving voltage controller 1832controls the driving voltage to a fourth driving voltage which is higherthan the reference voltage by 8%, and supplies the fourth drivingvoltage (ACT 111). The light emission controller 183 repeats ACT 102 toACT 111 from image data corresponding to a head line to image datacorresponding to a final line, and finishes the light emission controlbased on the image data corresponding to the final line (ACT 112, YES).When there is image forming processing for the next page, the lightemission control based on the image data for the next page is executed,and when there is no image forming processing for the next page, thelight emission control is ended.

FIG. 12 is a non-application example of the driving voltage controlaccording to the embodiment, and is illustrates a relationship betweenthe number of light emitting elements that emit light and the lightquantity decrease rate.

In FIG. 5, a connection example between the control board and the printhead was described. The power source section 102 illustrated in FIG. 5supplies the power source voltage VDDa to both ends of the print head 1via the harness 104. When the current increases as the number of lightemitting elements 131 that emit light increases, a voltage drop occursin the harness 104 and the wiring in the print head 1. As a result, thepower source voltage VDDa supplied from the power source section 102drops to a power source voltage VDDb in the print head 1. The voltagedrop in the wiring in the print head 1 becomes large, and the rate oflight quantity decrease at the center portion of the light emittingelement row 13 of the print head 1 becomes high. The higher the lightingrate, the higher the rate of light quantity decrease. The influence ofthis phenomenon on an image differs between the main scanning directionand the sub-scanning direction. As illustrated in FIG. 12, the lightquantity decrease (change) in the main scanning direction is smooth atany lighting rate. Therefore, the image density change due to the lightquantity change in the main scanning direction is inconspicuous.Meanwhile, since the light quantity change in units of lines depends onthe number of light emitting elements in that line, when a line with asmall number of light emitting elements and a line with a large numberof light emitting elements are adjacent to each other, a rapid lightquantity change will occur in the sub-scanning direction. For example,when a light quantity difference between the lines is equal to orgreater than 3%, a density difference becomes noticeable.

FIG. 13 illustrates an example of a relationship between a halftoneimage (when the number of light emitting elements that emit light isconstant) formed by the image forming apparatus according to theembodiment, and a voltage and a driving current supplied to the printhead in response to the formation of the halftone image.

As illustrated in FIG. 13, in forming each image line, when the numberof light emitting elements 131 that emit light constantly transitions,the driving current supplied to the print head 1 constantly transitions.When the driving current constantly transitions, the voltage supplied tolight emitting element 131 also constantly transitions, and does notchange in units of lines. When the voltage supplied to the lightemitting element 131 constantly transitions, the light emitting element131 emits a constant quantity of light. When the light emitting element131 emits a constant quantity of light, the image density is constant ineach image line (the density does not change in the sub-scanningdirection). Furthermore, the light quantity changes in the main scanningdirection, but as described above, the change rate is extremely smooth,and a level that cannot be recognized as a density difference isachieved.

FIG. 14 is a non-application example of the driving voltage controlaccording to the embodiment, and illustrates an example of arelationship between a halftone image (when the number of light emittingelements that emit light is not constant depending on the block) formedby the image forming apparatus, and a voltage and a driving currentsupplied to the print head in response to the formation of the halftoneimage. In addition, FIG. 14 illustrates a case where the sub-scanningdirection is divided into five blocks including first to fifth blocks,and the same image represents each block in order to make thedescription easy to understand. The halftone images of the first, third,and fifth blocks in FIG. 14 are the same as the halftone image in FIG.13. In the second and fourth blocks, a solid black image with a highprinting rate is disposed at the center portion of the halftone image ofFIG. 13, and a solid black area of the fourth block is greater than asolid black area of the second block.

As illustrated in FIG. 14, in forming each block image, the second andfourth blocks have a solid black part at the center portion thereof.Therefore, the number of light emitting elements 131 that emit lightincreases and the driving current supplied to the print head 1increases. When the driving current increases, a voltage drop occurs dueto a resistance component of the harness 104 and the wiring in the printhead 1, and the driving voltage supplied to the light emitting element131 decreases. When the driving voltage decreases, the light quantity ofthe light emitting element 131 decreases. In forming the images of thefirst, third, and fifth blocks, the numbers of light emitting elements131 that emit light are the same. Therefore, the light quantities of thelight emitting elements 131 are also the same, and the image densitiesof the first, third, and fifth blocks are the same. In forming the imageof the second block, the number of light emitting elements 131 that emitlight in correspondence to the solid black at the center portion of thesecond block increases, and thus the driving current increases. When thedriving current increases, the voltage supplied to the light emittingelement 131 decreases, and the light quantity of the light emittingelement 131 decreases. The halftones of both end portions of the secondblock are the same as the halftones of the first, third, and fifthblocks, but the light quantity of the light emitting element 131decreases, and thus the halftone image density of both end portionsdecreases. Therefore, although the halftone images at both ends of thesecond block are the same as the halftone images of the first block andthe third block, a density difference occurs at a block boundaryportion, and the halftones look different. In forming the image of thefourth block, the number of light emitting elements 131 that emit lightin correspondence to the solid black at the center portion of the fourthblock further increases, and thus the driving current further increases.When the driving current further increases, the voltage supplied to thelight emitting element 131 further decreases, and the light quantity ofthe light emitting element 131 further decreases. The halftones of bothend portions of the fourth block are the same as the halftones of thefirst, third, and fifth blocks and the halftones at both ends of thesecond block, but the light quantity of the light emitting element 131further decreases, and thus the halftone image density at both endportions further decreases. Therefore, although the halftone images atboth ends of the fourth block are the same as the halftone images of thethird block and the fifth block, a larger density difference than thatof the second block occurs at the block boundary portion, and thehalftones look different.

When the number of light emitting elements 131 that emit light in thismanner increases, the light quantity of the light emitting element 131decreases, the image density decreases, and the density change becomesnoticeable at the block boundary in the sub-scanning direction. Inaddition, the relationship between the light quantity change and theimage density change may be reversed depending on the developing method.

FIG. 15 is an application example of the driving voltage controlaccording to the embodiment, and illustrates an example of arelationship between a halftone image (when the number of light emittingelements that emit light is not constant depending on the block) formedby the image forming apparatus, and a voltage and a driving currentsupplied to the print head in response to the formation of the halftoneimage.

As illustrated in FIG. 15, in forming each image line, the drivingvoltage controller 1832 changes the voltage supplied to the print head 1according to the number of light emitting elements 131 that emit light.In forming each image line, when the number of light emitting elements131 that emit light is large, the driving voltage controller 1832increases the voltage to be supplied, and compensates for the voltagedrop that occurs in a wiring resistor due to the increased current. Whenthe voltage decrease is suppressed, the light quantity decrease issuppressed (the light quantity difference is suppressed to be equal toor less than 3%), and the density fluctuation can be made inconspicuous.

For example, in forming the images of the first, third, and fifthblocks, when the proportion of the light emitting elements 131 that emitlight exceeds 20% and is equal to or less than 40%, the driving voltagecontroller 1832 changes the driving voltage to a driving voltage whichis higher than the reference voltage by 2%. In forming the image of thesecond block, when the proportion of the light emitting elements 131that emit light exceeds 40% and is equal to or less than 60%, thedriving voltage controller 1832 changes the driving voltage to a drivingvoltage which is higher than the reference voltage by 4%. In forming theimage of the fourth block, when the proportion of the light emittingelements 131 that emit light exceeds 80%, the driving voltage controller1832 changes the driving voltage to a driving voltage which is higherthan the reference voltage by 8%.

FIG. 16 illustrates an example of an effect of suppressing the lightquantity decrease due to an increase in voltage applied to the opticalhead of the image forming apparatus according to the embodiment. Bycontrolling the driving voltage, the effect illustrated in FIG. 16 isobtained.

FIG. 16 illustrates an example in which the driving voltage is changedto a driving voltage which is higher than the reference voltage by 8%when the lighting rate of the light emitting elements 131 having thelargest light quantity decrease rate is 80% (refer to FIG. 12). When thedriving voltage is the reference voltage, the light quantity decreaseoccurs by approximately 6% at the end portion and approximately 10% atcenter portion of the print head 1. (same as FIG. 12)

On the other hand, when the driving voltage is changed to a voltagewhich is higher than the reference voltage by 8%, the light quantitybecomes +2% at the end portion and −2% at the center portion of theprint head 1 as illustrated in FIG. 16. At an intermediate positionbetween the end portion and the center portion, the light quantitybecomes ±0%. As the driving voltage controller 1832 controls the drivingvoltage according to the lighting rate (the number of elements that emitlight) of the light emitting elements 131, the light quantity fallswithin a predetermined range (±3%) with respect to the reference lightquantity (0%), and it is possible to prevent the occurrence of a densitydifference.

Here, an application example of control with respect to the lightingrate of 80% is illustrated, but the same effect can be obtained incontrol with respect to other lighting rates. Further, the control panel179 may set application or non-application of the driving voltagecontrol according to the input of application or non-application of thedriving voltage control. The non-volatile memory 177 stores the settingof application or non-application of the driving voltage control. Whenthe application of the driving voltage control is set, the controller174 and the light emission controller 183 detect the number of lightemitting elements 131 that emit light, and changes the driving voltageaccording to the proportion of the number of light emitting elements 131that emit light. When the non-application of the driving voltage controlis set, the controller 174 and the light emission controller 183 do notexecute the driving voltage control according to the proportion of thenumber of the light emitting elements 131 that emit light. When thedriving voltage control is applied, it is possible to prevent the lightquantity decrease as illustrated in FIG. 12 and the image qualitydeterioration as illustrated in FIG. 14. In addition, a powerconsumption amount can be suppressed by not applying the driving voltagecontrol.

FIG. 17 is a block diagram illustrating a modification example of thecontrol system of the image forming apparatus according to theembodiment. In the control system of the image forming apparatusillustrated in FIG. 10, the light emission controller 183 includes thedriving voltage controller 1832 which controls the driving voltage ofthe print head, but the control system of the image forming apparatusillustrated in FIG. 17 is different in that the controller 174 includesa driving voltage controller 1741. In the control system of the imageforming apparatus illustrated in FIG. 17, the light emitting elementnumber detection section 1831 detects the number of light emittingelements 131 that emit light according to the image data before thelight emitting element 131 emits light according to the image data, andoutputs the detection result to the driving voltage controller 1741. Thedriving voltage controller 1741 controls the driving voltage for drivingthe light emitting element 131 based on the detection result from thelight emitting element number detection section 1831. In addition, thedriving voltage control described in the embodiment is applicable toboth a monochrome image forming apparatus having a single print head anda color image forming apparatus having each print head corresponding toeach color. Further, a case where the light emitting element numberdetection and the driving voltage control are realized by software wasdescribed, but the light emitting element number detection and thedriving voltage control may be realized by hardware.

The above-described image forming apparatus according to the embodimentcan suppress the image quality deterioration by increasing the drivingvoltage according to the increase rate of the number of light emittingelements that emit light when the number increases corresponding to eachimage line.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of invention. Indeed, the novel apparatus and methods describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the apparatus andmethods described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

What is claimed is:
 1. An image forming apparatus comprising: aplurality of light emitting elements arranged in at least one row; adetector configured to detect a number of light emitting elements thatemit light according to image data; a controller configured to control adriving voltage for driving the light emitting elements based on thenumber of light emitting elements detected; and an image forming unitconfigured to form an image based on light emission of the lightemitting elements according to the image data.
 2. The image formingapparatus of claim 1, wherein the light emitting elements are arrangedin a plurality of rows.
 3. The image forming apparatus of claim 2,wherein: the detector is configured to detect a proportion of the lightemitting elements that emit light in units of one or more of the rows;and the controller is configured to change the driving voltage based onthe proportion.
 4. The image forming apparatus of claim 3, wherein thecontroller is configured to change the driving voltage from a referencevoltage to a first voltage higher than the reference voltage when theproportion exceeds a first threshold.
 5. The image forming apparatus ofclaim 4, wherein the controller is configured to change the drivingvoltage from the reference voltage to a second voltage higher than thefirst voltage when the proportion exceeds a second threshold higher thanthe first threshold.
 6. The image forming apparatus of claim 4, whereinthe controller is configured to set the driving voltage to the referencevoltage when the proportion is equal to or less than the firstthreshold.
 7. The image forming apparatus of claim 1, wherein thedetector is configured to detect the number of light emitting elementsthat emit light according to the image data before the light emittingelements emit light.
 8. An image forming apparatus comprising: aplurality of print heads, each of the print heads configured to includea plurality of light emitting elements arranged in at least one row,each of the print heads corresponding to a color; a detector configuredto detect a number of light emitting elements that emit light accordingto image data corresponding to a color for each of the print heads; acontroller configured to control a driving voltage for driving the lightemitting elements based on the number of light emitting elements thatemit light in each of the print heads; and an image forming unitconfigured to form an image based on light emission of the print headsaccording to the image data.
 9. The image forming apparatus of claim 8,wherein: the detector is configured to detect a proportion of the lightemitting elements that emit light in units of one or more of the atleast one row; and the controller is configured to change the drivingvoltage based on the proportion.
 10. The image forming apparatus ofclaim 9, wherein the controller is configured to change the drivingvoltage from a reference voltage to a first voltage higher than thereference voltage when the proportion exceeds a first threshold.
 11. Theimage forming apparatus of claim 10, wherein the controller isconfigured to change the driving voltage from the reference voltage to asecond voltage higher than the first voltage when the proportion exceedsa second threshold higher than the first threshold.
 12. The imageforming apparatus of claim 10, wherein the controller is configured toset the driving voltage to the reference voltage when the proportion isequal to or less than the first threshold.
 13. The image formingapparatus of claim 8, wherein the detector is configured to detect thenumber of the light emitting elements that emit light according to theimage data before the light emitting elements emit light.
 14. A lightemitting control system for an image forming apparatus, the lightemitting control system comprising: a plurality of light emittingelements arranged in at least one row; a detector configured to detect anumber of light emitting elements that emit light according to imagedata; and a controller configured to control a driving voltage fordriving the light emitting elements based on the number of lightemitting elements detected.
 15. The light emitting control system ofclaim 14, wherein the light emitting elements are arranged in aplurality of rows.
 16. The light emitting control system of claim 15,wherein: the detector is configured to detect a proportion of the lightemitting elements that emit light in units of one or more of the rows;and the controller is configured to change the driving voltage based onthe proportion.
 17. The light emitting control system of claim 16,wherein the controller is configured to change the driving voltage froma reference voltage to a first voltage higher than the reference voltagewhen the proportion exceeds a first threshold.
 18. The light emittingcontrol system of claim 17, wherein the controller is configured tochange the driving voltage from the reference voltage to a secondvoltage higher than the first voltage when the proportion exceeds asecond threshold higher than the first threshold.
 19. The light emittingcontrol system of claim 17, wherein the controller is configured to setthe driving voltage to the reference voltage when the proportion isequal to or less than the first threshold.
 20. The light emittingcontrol system of claim 14, wherein the detector is configured to detectthe number of the light emitting elements that emit light according tothe image data before the light emitting elements emit light.